Adaptive shifts in honey bee (Apis mellifera L.) guarding behavior support predictions of the acceptance threshold model

The acceptance threshold model predicts that in a fluctuating environment a recognition system should be adaptive rather than fixed. In particular, discriminating individuals, such as guards at a nest entrance, should be less permissive to conspecifics when both the frequency of non-nest-mate contact and the cost of accepting non-nest mates is high. We tested these predictions by studying honey bee guarding during a period in which nectar conditions changed from dearth to abundance. Initially, during nectar dearth, individual guards accepted 80% of introduced nest mates and 25% of non-nest mates. As nectar conditions improved, both the intensity of robbing and guarding and the cost of non-nest-mate acceptance declined. In response, individual guards became more permissive to nest mates and non-nest mates until eventually an "accept-all" threshold occurred-all nest mates and non-nest mates were accepted. These data are consistent with a shifting acceptance threshold and provide the first field data to support the model. A simple linear relationship occurred between the number of guards and the number of fights, 9:1, observed at the hive entrance, suggesting that guarding may be regulated by intruder intensity or otherwise regulated in an adaptive manner.